The present invention generally relates to a sampler mechanism for use in gas chromatography and, in particular, relates to a sampler mechanism adapted for use as a head space sampler wherein samples are automatically provided under comparable thermal conditions.
In a closed sample vessel, a state of equilibrium exists in the head space above a liquid sample wherein the partial pressures of the individual sample components are proportional to the concentrations thereof in the liquid sample. In a sampler operating on the head space method, a metered volume from the head space of the sample vessel is conveyed to the inlet of a gas chromatograph. The composition of the liquid sample in the sample vessel is thereafter derived from the analysis of the gas composition of the head space sample.
In one known sampler (German Pat. No. 1 284 660), sample vessels are sealed by a self-sealing diaphragm or septum. A sample is obtained by piercing the septum with a needle which is connected to the entrance of a gas chromatographic separating column. The entrance of the separating column is in turn connected to a carrier gas conduit, the flow of which is controlled by a solenoid valve. When the solenoid valve is open, the carrier gas pressure at the entrance of the separating column is transferred to the head space of the sample vessel via the needle which acts as a capillary such that an elevated pressure is built up therein. The partial pressures of the samples, however, are unaffected. The pressure at the entrance of the separating column breaks down when the carrier gas conduit is shut off. Thus, carrier gas plus sample vapor flow from the head space to the inlet of the gas chromatograph at the entrance of the separating column. The sample volume is determined by the time interval during which the solenoid valve is shut off.
To obtain reproducible results and sufficient vapor pressures, the sample vessels are usually controllably and reproducibly maintained at an elevated temperature. A variety of mechanisms have been developed for this purpose. For example, German Pat. No. 1 297 904 discusses such a sampler wherein a turntable having a liquid bath for receiving a plurality of sample vessels is arranged for being stepwise advanced. In this manner, the sample vessels are individually positioned below a stationary needle. Together with the liquid bath, the turntable is guided for vertical movement and is lifted to individually push sample vessels onto the needle to pierce the septum of the sample vessel.
In another design (German Offenlegungsschrift No. 2 818 251), a turntable consists of a stationary axle and a base plate. A metal block, heatable by an electric heater, serves as a thermostating means, which metal block is rotatably mounted about the stationary axle. The metal block having the axle as its center includes a circular array of axial through bores. A base plate closing the axial through bores from below has an aperture for pushing a sample vessel into one of the axial through bores aligned with the aperture. A cover, arranged to be pushed aside resiliently, is provided in front of the aperture.
In conventional samplers, sample vessels are manually set into a turntable and into the through bores of the rotatable metal block, respectively. Therefrom, the following problems result. It is possible to insert a plurality of sample vessels into the turntable and into the metal block, respectively, at a time. These samples might then automatically be sampled by means of an automatic control, through which the turntable may be stepwise advanced and may be lifted towards the needle. The number of the samples fed in this way is limited to the number of sample positions available in the turntable and in the metal block, respectively. Furthermore, the thermostating times for the different samples are different with this mode of operation. This can result in faulty measurements of samples in which thermal equilibrium is reached only very slowly and such a stationary state is not reached at the moment of the sample transfer. In this case, with different thermostating times, there are no comparable conditions for the different samples. It can also happen that samples chemically change due to the heating either by thermal decomposition or by reaction of their components with each other. In this case also, comparable conditions are not obtained if the thermostating times are different for the different samples. If this is to be avoided with conventional samplers, the samples must be supplied continuously at the same rate as the gas chromatographic analysis is carried out and the turn table is advanced.